1. Field of the Invention
The present invention relates to a rotary driving device for a press machine driven by a striker of the press machine, and more specifically a rotary driving device suitable for use with a turret punch press being mounted on a die mounting station thereof.
2. Description of the Related Art
Examples of the rotary driving device for a press machine (which is usable being mounted on a die mounting station of a turret punch press, for instance) are already disclosed in Japanese Published Unexamined (Kokai) Patent Application Nos. 4-51323, 4-115816, 4-115817, etc. Each of these rotary driving devices comprises: a cylindrical main body fitted to a die mounting hole formed in a die mounting station; a push head member provided for the main body reciprocatingly movably in an axial direction thereof and driven in the axial direction by a striker of the press machine; a male thread axle member in mesh with female threads formed in the main body and rotatable when moved in an axial direction thereof by the push head member; a lead thread member in mesh with a lead nut and rotatably supported by the main body, for holding a rotary driven tool; and a coupling section for coupling the male thread axle member and the lead thread member in torque transmission relationship and relative axial displacement relationship with respect to each other so as to absorb a difference in axial speed between both the members.
In the above-mentioned prior art rotary driving device for a press machine, since a strong impact force is generated the instant that a rotary driven tool is brought into pressure contact with work; that is, the instant that the linear motion is switched to a rotary motion, there exists a problem in that the work is inevitably damaged by the energy or the pressure thereof, so that pressure traces are formed on the work.
Further, the above-mentioned impact force is transmitted to a rotary thread (a ball screw or a roller screw) and a nut, there exists another problem in that this shock exerts a bad influence upon these screw and nut.
FIGS. 1A to 1C show the relationship between the sliding resistance of a rotary thread, in which FIG. 1A shows that obtained when a rotary thread of 4 mm stroke per revolution is used; FIG. 1B shows that obtained when a rotary thread of 3 mm stroke per revolution is used; and FIG. 1C shows that obtained when a rotary thread of 2 mm stroke per revolution is used. In these drawings, E denotes a point when the rotary driven tool is brought into pressure contact with the work; F denotes the bottom dead center of the press; G denotes an impact force; and H denotes a rotary resistance, respectively. As shown in FIGS. 1A to 1C, since the impact force increases beyond an allowable range in the case of the rotary threads of 3 and 2 mm stroke per revolution, at least the rotary thread of 4 mm stroke per revolution so far must be used. Further, when the velocity of linear motion increases, the impact force also increases in proportion thereto.
On the other hand, in the press machine, a return spring is necessary to return the push head member in a direction opposite to the striking direction of the striker. In the prior art rotary driving device, this return spring is of gaseous spring type such that a piston member is slidably fitted into a cylinder boredirectly formed in the main body by cutting processing and further a gaseous spring chamber is filed with a compressive fluid (e.g., nitrogen gas) supplied from a separate gas tank.
In the prior art gas spring as described above, since the cylinder bore is directly formed in the main body of the rotary drive device, there exists a problem in that it is difficult to secure the air tightness of the gaseous spring chamber, so that the gas in the spring chamber inevitably leaks. Here, once the gas in the gaseous spring chamber leaks, since the spring force of the gaseous spring is lowered, it has been necessary to often replenish the gaseous spring chamber with gas to maintain a required gas pressure, thus resulting in a problem in that it is impossible to realize an unmanned press machine operated for many hours at night, for instance.
In addition, since the separate gas tank must be fitted into a die mounting hole different from the main body of the rotary driving device, the rotary driving device occupies two die mounting stations in total in the case of a die mounting station of "1 (1/4)" size, for instance. As a result, the prior art rotary driving device can be used only with the turret punch press provided with a multi-track (e.g., 3 tracks) die mounting station, thus causing a limitation in use of the rotary driving device with the press machine.